Variable capacity rotary screw compressor having variable high pressure suction fluid inlets

ABSTRACT

A variable capacity multiple inlet rotary screw compressor having a pair of helical rotors mounted within a housing for compressing fluid drawn from a low pressure suction fluid chamber and a high pressure suction fluid chamber and discharging to a discharge chamber. A slide valve is provided in the housing to control the capacity of the compressor by changing the point of low pressure suction fluid cut off to the rotors. The point of high pressure suction fluid admission being changed in synchronism with the movement of the slide valve to connect the working chambers of the rotors to the high pressure fluid suction fluid chamber simultaneously with or just after low pressure suction fluid cut off.

Kocher et a1.

1 1 Mar. 4, 1975 VARIABLE CAPACITY ROTARY SCREW COMPRESSOR HAVINGVARIABLE HIGH PRESSURE SUCTION FLUID INLETS Inventors: Erich J. Kocher,Milwaukee;

Whitney 1. Grant, Muskego, both of Wis.

Vilter Manufacturing Corporation, Milwaukee, Wis.

Filed: Mar. 8, 1974 Appl. No.: 449,263

Assignee:

US. Cl 418/15, 418/159, 418/201 Int. Cl. F0lc 1/16 Field of Search418/15, 159,201,202,

References Cited UNITED STATES PATENTS 5/1963 Nilsson et a1. 418/201 X3/1969 Schibbye 418/201 X 12/1970 Linneken 418/201 X 9/1973 Persson eta1 418/159 X FOREIGN PATENTS OR APPLICATIONS 663.144 5/1963 Canada418/201 Primary Examinew-C. J. I-Iusar Assistant E.\'aminer-LeonardSmith Attorney, Agent, or Firm-James E. Nilles [57] ABSTRACT A variablecapacity multiple inlet rotary screw compressor having a pair of helicalrotors mounted within a housing for compressing fluid drawn from a lowpressure suction fluid chamber and a high pressure suction fluid chamberand discharging to a discharge chamber, A slide valve is provided in thehousing to control the capacity of the compressor by changing the pointof low pressure suction fluid cut off to the rotors. The point of highpressure suction fluid admission being changed in synchronism with themovement of the slide valve to connect the working chambers of therotors to the high pressure fluid suction fluid chamber simultaneouslywith orjust after low pressure suction fluid cut off.

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f 6 6 4 M 9 9 9 L 0 l x 9) 8 8 W PATENTEU 4 I975 SHEET 3 0F 5 VARIABLECAPACITY ROTARY SCREW COMPRESSOR HAVING VARIABLE HIGH PRESSURE SUCTIONFLUID INLETS BACKGROUND OF THE INVENTION Multiple suction variablecapacity screw compressors utilize a slide valve to vary the capacity ofthe compressor between minimum and maximum capacity positions. The slidevalve is used to control the low pressure suction fluid inlet producinga change in the pressure at the point of admission of the high pressuresuction fluid. In a copending application Ser. No. 403,195, filed Oct.3, 1973 and entitled Variable Capacity Rotory Screw Compressor", thepoint of injection of the secondary or high pressure suction fluid ismaintained constant with respect to the low pressure suction fluid cutoff in order to maintain the constant pressure at the point of admissionof the high pressure suction fluid. This was accomplished by introducingthe high pressure suction fluid at a fixed distance from the lowpressure suction fluid cut off.

SUMMARY OF THE PRESENT INVENTION The multiple inlet variable capacityscrew compressor of the present invention provides a change in the pointof admission of the high pressure suction fluid vapor with a change inthe position of the slide valve in order to admit high pressure suctionfluid at the point of low pressure suction fluid cut off. In oneembodiment of the invention, this is achieved by advancing or retractingthe point of injection of the high pressure suction fluid vapor withrespect to the position of the working chamber between the rotors. Anelectrohydraulic sensing system is used to sense the position of theslide valve and to open or close angularly offset plug valve assemblies.In the alternate embodiment of the invention, a continuing change in thepoint of admission of the high pressure suction fluid is provided with acontinuing change in the position of the slide valve. Control valveassemblies are provided in each of the rotors of the compressor whichare controlled by an electro-hydraulic circuit that maintains asynchronized relationship between the position of the slide valve andthe control valve assemblies through an electrically balanced circuit.

DRAWINGS FIG. I is a plan view partially in section of the variablecapacity compressor according to the present invention;

FIG. 2 is a section view taken on line 2--2 of FIG. 1 showing the inletpassages from the low pressure suction fluid inlet chamber to theworking chambers of the COITIPI'CSSOI';

FIG. 3 is a view taken on line 3-3 of FIG. 1 showing the connection ofthe high pressure suction fluid chamber and the discharge chamber to theworking chambers of the compressor;

FIG. 4 is a section view taken on line 44 of FIG. 3 showing the positionof the slide valve and the plug valve assemblies in the minimum capacityposition;

FIG. 5 is a plan view similar to FIG. 1 showing the hydraulic circuitfor controlling the plug valve assemblies;

FIG. 6 is a plan view similar to FIG. 5 showing the slide valve in themaximum capacity position and the hydraulic control circuit in themaximum capacity position;

FIG. 7 is a plan view partially in section showing another embodiment ofthe variable capacity compressor of this invention;

FIG. 8 is a view taken on line 8-8 of FIG. 7 showing the connection ofthe low pressure suction fluid chamber to the working chambers of therotors;

FIG. 9 is a view taken on line 9-9 of FIG. 7 showing the connection ofthe discharge chamber to the working chambers of the rotors;

FIG. 10 is a view taken on line 10-10 of FIG. 7 showing the connectionof the high pressure suction fluid chamber to the rotors;

FIG. 11 is a perspective view of the control cylinder which is used tocontrol high pressure suction fluid admission to the rotors;

FIG. 12 is a section view taken on line 12-12 of FIG. 9 showing thecontrol valve assembly for controlling the admission of high pressuresuction fluid to the grooves in one of the rotors; and

FIG. 13 is a view similar to FIG. 12 showing the control valve assemblyin the maximum capacity position; and

FIG. 14 is a schematic diagram of an electrohydraulic circuit forcontrolling the movements of the control assemblies.

DESCRIPTION OF THE INVENTION The variable capacity multiple inlet rotaryscrew compressor 10 of this invention generally includes a pair ofoppositely rotating constant mesh rotors l2, 14 which define workingchambers 16 within a housing 18. The working chambers 16 are in fluidcommunication with a low pressure suction fluid inlet chamber 20 at oneend of the housing 18 and to a high pressure suction inlet chamber 22 atthe other end of the housing 18. Fluid drawn from the chambers 20 and 22is compressed in the working chambers 16 is discharged to a dischargechamber 24.

The capacity of the compressor 10 is adjusted by means of a slide valve26 which controls the low pressure suction fluid cut off. The admissionof the high pressure suction fluid is varied so that communication withthe working chambers 16 occurs simultaneously with or just after lowpressure cut off. In FIGS. 1 through 6, this is achieved by means of anumber of plug valve assemblies 28. The timing of the operation of theplug valve assemblies 28 in relation to the position of the slide valve26 is controlled by means of an electro-hydraulic circuit 30 whichincludes a cam operated switch assembly 32 that is connected to theslide valve 26.

In a variable capacity compressor of the type contemplated herein, asthe slide valve 26 is moved between maximum and minimum capacitypositions, the plug valve assembly 30 must be opened or closed so thathigh pressure suction fluid cannot bypass through the working chambers16 between. the rotors l2 and 14 back to the low pressure suctionchamber 20. More particularly and referring to FIGS. 5 and 6, it shouldbe noted that as the rotors 12 and 14 intermesh to form the workingchambers 16, the working chambers will initially register with the lowpressure suction chamber 20, then the high pressure suction chamber 22and finally discharge through the discharge chamber 24. In order toobtain maximum efflciency, the high pressure suction fluid chamber 22should be in communication with the working chambers 16 simultaneouslywith or just after low pressure cut off but prior to pressure build upin the working chambers 16. As the point of low pressure cut off ischanged by moving slide valve 26 to reduce capacity, the point ofadmission of high pressure suction fluid to the working chambers 16 mustbe varied so that the high pressure suction fluid enters the workingchambers 16 in accordance with the new low pressure suction cut-off.

THE VARIABLE CAPACITY COMPRESSOR OF FIGS. 1-6

More specifically, the housing 18 includes two intersecting bores 34 and36 closed at each end by end walls 38 and 40. The low pressure suctionchamber 20 is provided between the end wall 38 and outside wall 39. Thehigh pressure suction chamber 22 is provided between the end wall 40 andoutside wall 41. A low pressure suction inlet port 46 is connected tothe chamber 20. A high pressure suction inlet port 48 and a dischargeport 50 are connected to the high pressure suction chamber 22 anddischarge chamber 24, respectively. Chamber 22 is separated fromdischarge chamber 24 by means of a partition 52.

The low pressure suction inlet chamber 20 is connected to the workingchambers 16 within the housing 18 by means of a pair of arcuate slots 54provided in the end wall 38. The high pressure suction inlet chamber 22is connected to the working chambers 16 by ports 56a and 56b in end wall40. The discharge chamber 24 is connected to the working chambers 16through arcuate slots 58 in end wall 40.

The rotors 12 and 14 are mounted for rotation on shafts 55 which aresupported in a parallel spaced relation by end walls 38 and 40. Eachrotor includes a number of helical lands 60 and grooves 62. The wrapangle of the land 60 on the male rotor 12 is approximately 270 and thewrap angle of the land 60 on its female rotor 14 is approximately 180.The rotors l2 and 14 are positioned within the working cylinders definedby the end walls 38 and 40 and the parallel bores 34 and 36. Thecompressor can be driven in a conventional manner by an electric motor64 connected to a shaft 57 on rotor 14.

COMPRESSOR CAPACITY CONTROL The capacity of the compressor 10 is variedby means of the slide valve 26 which is positioned in a bore 66 providedin the housing 18 in a parallel relation to the bores 34 and 36. Theslide valve 26 forms a movable wall for portions of the wall of thebores 34 and 36 in the housing 18 and is axially movable in the bore 66from the minimum capacity position shown in FIG. 5 'to a maximumcapacity position shown in FIG. 6. The slide valve 26 can be moved tothe maximum capacity position by means ofa hydraulic piston and cylinderassembly 70 connected to one end of the slide valve 26 by a shaft 67 andis normally biased to the minimum capacity position by means of a spring68.

The point or timing of admission of low pressure suction fluid or vaporinto the working chambers of the rotors l2 and 14 is determined by theposition of the face or end 72 of the slide valve 26. In the minimumcapacity position of the slide valve 26, the end 72 of the slide valveis spaced from end wall 38 connecting a portion of the bores 34 and 36for the rotors 12 and 14 to the bore 66 for slide valve 26. The workingchambers 16 of the rotors will not close until the point of contact ofthe helical land 60 of the rotor 12 seats in groove 62 of rotor 14 andpasses the end 72 of the slide valve 26. The stroke of the compressor atthe minimum capacity position will be equal to the distance of the end72 of the slide valve 26 to the end wall 40 at the discharge end of therotors 12 and 14.

HIGH PRESSURE SUCTION FLUID ADMISSION The location of the point ofadmission or introduction of the high pressure suction fluid or vaporinto the working chambers 16 between the rotors l2 and 14 is determinedby opening or closing the plug valve assemblies 28a and 28b provided inthe high pressure suction chamber 22. In this regard, each of the plugvalve assemblies 28 includes a valve member 74 which is positioned formovement into a closing position with respect to the ports 56 in endwall 40. The valve members 74 are actuated by means of double actingpiston and cylinder assemblies 76 and 78 provided on the end of thehousing 18. In the minimum capacity position of the slide valve 26,(FIG. 5) the high pressure suction fluid must be admitted into thechambers 16 as they are closed by the end 72 of the slide valve 26.Since only a portion of the working chambers 16 formed between therotors 12 and 14 is used in the minimum capacity position of the slidevalve 26, the outer ports 56a should be opened and the inner ports 56bclosed. As the slide valve 26 is moved toward the maximum capacityposition the full length of the working chambers 16 defined by therotors l2 and 14 will be used and the ports 56a closed and the ports 56bopened to allow the high pressure suction fluid to enter the workingchambers 16 at an earlier point in the rotory motion of the rotors 12and 14.

Means are provided for synchronizing the opening and closing of the plugvalve assemblies 28a and 28b with the position of the slide valve 26.Such means is in the form of an electro-hydraulic circuit 30 shown inFIGS. 5 and 6. In the minimum capacity position of the slide valve asseen in FIG. 5, the plug valve assemblies 280 should be in the openposition and the plug valve assemblies 28b should be in the closedposition.

In the maximum capacity position (FIG. 6) of the slide valve 26, theplug valve assemblies 28a should be closed and the plug valve assemblies28b should be opened. This is accomplished by means of the solenoidactuated valves 80 and 82. The solenoid valve 80 is connected to asource of hydraulic fluid 84 under pressure. The solenoid valve 82 isconnected to a reservoir 86. In the minimum capacity position of theslide valve 26 shown in FIG. 5, fluid under pressure will flow throughthe solenoid valve 80 and lines 88 and 90 to the cylinders 78 for theplug assemblies 280 to move the pistons 76 to the open position. Fluidunder pressure will also flow through line 92 and 94 to the cylinders 78for the plug valve assemblies 28b to move the valve members 74 to theclosed position. Fluid discharging from the cylinder 76 in the plugassemblies 28a will flow through the lines 96 and 98 to the solenoidvalve 82 for discharge to the reservoir 86. Fluid discharging from thecylinder 78 for the plug assemblies 28bwill discharge through lines 100and 98 to the solenoid valve 82 for discharge to the reservoir 86.

In the maximum capacity position of the slide valve 26 shown in FIG. 6,the solenoid valves 80 and 82 are reversed to open plug valve assemblies28b and close plug valve assemblies 28a. In this position, fluid underpressure will flow through solenoid valve 80 and lines 96 to cylinder 76and lines 98 and 100 to cylinders 78. Fluid discharging from cylinders78 will flow through lines 94 and 92 and from cylinders 76 through lines90 and 88 through solenoid valve 82 to reservoir 86.

The solenoid valves 80 and 82 are controlled by means of the camactuated switch assembly 32 which is connected to respond to themovement of the slide valve to open or close the electrical circuit tothe solenoid valves 80 and 82. The assembly 32 includes a switch 102 anda cam 104. In FIG. 5, the cam 104 is shown in the minimum capacityposition with the switch 102 open. Rotation of the cam 104 to the maximum capacity position of the slide valve as seen in FIG. 6 will closeswitch 102 and energize the solenoid valves 80 and 82.

CONTINUOUS HIGH PRESSURE SUCTION FLUID CONTROL In the alternateembodiment of the invention (FIGS. 7 through 14), a variable capacitycompressor 106 is shown having a continuous high pressure suction fluidcontrol. This is accomplished by utilizing a pair of control valveassemblies 108 mounted within rotors 110 and 112 to control theadmission of high pressure suction vapor or fluid to the workingchambers 114 of the rotors 110 and 112.

In this regard, the compressor 106 includes acompressor housing 116having a pair of cylindrical bores 118 and 120 closed at one end bymeans ofa low pressure suction fluid or vapor housing 122 and at theother end by means of a high pressure fluid or vapor discharge housing124. A high pressure suction fluid or vapor housing 126 is mounted onthe discharge housing 124.

Fluid or vapor is pumped from a low pressure suction chamber 130 inhousing 122 and a high pressure suction fluid or vapor chamber 132 inhousing 126 to a discharge chamber 134 in discharge housing 124 by therotors 110 and 112. The capacity of the compressor 106 is adjusted bymeans of a slide valve 136 which controls the low suction fluid pressurecut off as described above. High pressure suction fluid admission iscontrolled by means of the control valve assemblies 108. The timing ofthe operation of the control valve assemblies 108 is controlled by meansof an electrohydraulic circuit 138 which synchronizes the position ofthe control valve assemblies 108 with the slide valve 136.

More particular, the low-pressure suction fluid housing 122 includes aninside wall 140 and an outside wall 142 spaced apart to define thechamber 130. The housing 122 is connected to the compressor housing 116by a number of bolts 144. Fluid communication between the chamber 130and the housing 116 is provided by means of a number of arcuate slots146 provided in the end wall 140 as seen in FIG. 8. A pair of rotorbearing openings 148 and 150 are provided in the inside end wall 140.Low pressure suction fluid is admitted to the chamber 130 through aninlet port 152.

The discharge housing 124 provides fluid communication between thedischarge chamber 134 and the compressor housing 116 by means of a pairof arcuate recesses 154 provided in the end face 156 of the housing 124.Fluid is discharged from chamber 134 through port 164 in housing 116.The housing 124 is mounted on the compressor housing 116 by means ofbolts 158.

A pair of rotor bearing openings 160 and 162 are provided in thedischarge housing 124 in axial alignment with the rotor openings 148 and150, respectively, in housing 122.

The high pressure suction fluid or vapor housing 126 includes a pair ofthreaded openings 166 which are axially aligned with openings 160 and162. An enlarged bore 170 is provided in each of the openings 160 on theinside surface of the housing 126. A pair of diametrically oppositeslots 172 are provided in each of the bores 170. The bores 170 areintersected by the chamber 132. High pressure suction fluid or vapor isadmitted to chamber 132 through an inlet port 175.

The rotors and 112 are mounted for rotation within the housing 116 andcooperate to provide working chambers 114 for compressing fluid admittedfrom the low pressure suction chamber and the high pressure suctionchamber 132. Rotor 110 is journalled for rotation in bearings 174provided in the opening 148 in the end wall and the opening in thedischarge housing 124. The rotor 112 is journalled for rotation inbearings 176 provided in the opening 150 in the end wall 140 and theopening 162 in the discharge housing 124. The rotors 110 and 1112 can bedriven by any suitable means such as a motor connected to a shaft 178provided on the end of the rotor 112. Rotor 110 includes an axiallyextending opening 180 which is closed at one end by end wall 182 and isconnected to the grooves in the rotor by radially extending ports 184.Rotor 112 is similar to rotor 110 and includes an axial bore 180connected to the grooves of the rotor by ports 184. Axial movement ofthe rotors 110 and 112 is prevented by twin ball thrust bearings 113.The rotors 110 and 112 are sealed by spring biased seals 115.

LOW PRESSURE SUCTION FLUID CONTROL The point of admission of lowpressure suction fluid or vapor into the working chambers 114 betweenthe rotors 110 and 112 is controlled by means of the slide valve 136 inthe same manner as described in the embodiment shown in FIGS. 1 through6. In this regard, the point of low pressure suction fluid cut off isdetermined by the position of the end 137 of the slide valve 136. In theminimum capacity position of the slide valve 136, the end 137 will bespaced from the end wall 140 connecting a portion of the bores 118 and120 to the bore 134 for the slide valve 136. The working chambers 114will not close until the point of contact of the lands and grooves ofthe rotors 110 and 112 pass the end 137 of the slide valve 136. Thestroke of the compressor at the minimum capacity position will be equalto the distance of the end 137 of the slide valve 136 to the face 156 ofthe discharge housing 124. The slide valve 136 is moved in substantiallythe same manner as shown in FIGS. 1 and 2.

HIGH PRESSURE SUCTION FLUID CONTROL ASSEMBLIES High pressure suctionfluid is admitted to the working chambers 114 through the blind bore 180and ports 184 provided in the rotors 110 and 112. The timing ofadmission of the high pressure suction fluid or vapor from the highpressure chamber 132 is controlled by means of the control assemblies108 provided in the bores 180 in rotors 110 and 112. In this regard,each of the control assemblies 108 includes a hollow cylinder 186 asseen in FIG. 11. The cylinder 186 is pro vided with a central bore 190,an axially extending arcuate alot 188 and an opening 192 in end wall194. A pair of axially extending guide bars or rods 196 are provided atone end of the cylinder 186. Fluid communication is provided from thebore 190 through the wall of the cylinder 186 by means of the arcuateslot 188. The cylinder 186 is positioned for axial movement within thebore 180 of the rotor 110 with the slot 188 aligned with the radialports 184 in the rotor 110. The cylinder 186 is prevented from rotatingin the bore 180 by means of the rods 196 which are aligned with theslots 172 in bore 170.

The cylinder 186 is biased to a minimum capacity position by means of aspring 198 provided within the bore 190 of the cylinder 186 and bearingagainst the housing 126. In the maximum capacity position shown in FIG.13, the end l88b of the slot 188 will be aligned in the radial plane ofports 184. In the minimum capacity position shown in FIG. 12, the end1880 will be moved beyond the radial plane of the ports 184 to cut offthe high pressure suction fluid.

Means are provided for moving the cylinder 186 axially in the bore 180of the rotor 110. Such means is in the form of a hollow tube 200 whichextends through the bore 190 and the opening 192 in the end wall 194 ofthe cylinder 186. The tube 200 is sealed in the opening 192 by means ofan O-ring seal 202. The tube 200 is retained in a fixed position in thecylinder 186 by means of a pressure seal 204 provided in the opening 166in the housing 126 and a lock nut or cap 206 which is threadedlyreceived in the threaded opening 166. Hydraulic fluid is admitted to thetube 200 through an inlet port 199 and discharged through a port 201.I-Iydraulic fluid under pressure is admitted to the bore 180 throughtube 200 into the space between end wall 194 and the bore 180. When thepressure of the fluid builds up sufficiently to overcome the force ofthe spring 198, the cylinder will move axially in the bore 180.

The cylinder 186 is lubricated for free movement in the bore 180 bymeans of a passage 208 provided in the side wall of the cylinder 186 anda number of ports 210. As hydraulic fluid is admitted into the bore 180it will flow through the passage 208 and the ports 210 into the spacebetween the cylinder 186 and the walls of the bore 180.

ELECTRO-HYDRAULIC CONTROL Means are provided for synchronizing theposition of the cylinder 186 with the position of the slide valve 136.Referring to FIG. 14, a schematic electrohydraulic diagram is shown forsynchronizing the movements of the cylinders 186 with the position ofthe slide valve 136. The electrohydraulic circuit includes an electricalcircuit 212 which is used to control solenoid valves 214 and 216 in thehydraulic circuit 138.

More particularly, the electric circuit 212 includes a firstpotentiometer 220 which is connected to the slide valve 136 by means ofa rod 221 and is used to convert the mechanical movement of the slidevalve 136 to an electrical signal. The electric signal is transferredthrough electric lines 222, 224 and 226 to a pair of bridge balancingrelays 228 and 230 through lines 223, 225 and 227. The bridge balancingrelays 228 and 230 are connected to a pair of potentiometers 232 and234, respectively. The potentiometers 232 and 234 are connected to thecylinders 186 by rods 235 and convert the mechanical position of thecylinders 186 to an electrical signal which is transmitted to therespective bridge balancing relays 228 and 230.

Each of the bridge balancing relays 232 and 234 is connected to an oilsupply solenoid valve 214 and to an oil drain solenoid valve 216. Eachof the oil supply solenoid valves 214 is connected to the inlet line 199to the tubes 200. Each of the oil drain solenoid valves 216 is connectedto the outlet port 201 to the tubes 200. The oil supply valves 214 areused to control the flow of hydraulic fluid from a source 236 ofhydraulic fluid under pressure such as a lube oil pump. Each of the oildrain solenoid valves 216 is used to control the discharge of hydraulicfluid from the tube 200 to the suction chamber 130.

In operation, both of the valves 214 and 216 are normally closed whenthe slide valve 136 is in the minimum capacity position. When the slidevalve 136 is moved from the minimum capacity postion to an intermediateposition or to the maximum capacity position, the potentiometer 220 willmove a corresponding amount indicating the mechanical movement of theslide valve and providing an electrical signal to each of the bridgebalancing relays 228 and 230. The initial change in the electricalsignal will unbalance the relays 228 and 230 opening the oil supplysolenoid valves 214 and allowing hydraulic fluid to flow into the tubes200 to move the cylinders 186 toward the maximum capacity position. Themovement of the cylinders 186 will be sensed by the potentiometers 232and 234 providing a signal of the postion of the cylinder 186 to thebridge balancing relays 228 and 230. When the potentiometer 220 for theslide valve comes to a stop, the potentiometers 232 and 234 willcontinue to move until the signal from the potentiometers 232 and 234 isbalanced with the signal from the potentiometer 220 in the relays 228and 230. When the signals are balanced, the solenoid valve 214 willclose.

When the slide valve 136 is moved in the opposite direction, themovement of the potentiometer 220 will again produce a signal to thebridge balancing relays 232 and 234 which will cause the valves 216 toopen allowing fluid to flow from the tube 200 to the low pressurechamber through lines 238 or to a reservoir. The cylinders 186 will movetoward the minimum capacity position due to the bias of the spring 198moving the potentiometers 232 and 234 until the signal is balanced withthe signal of the potentiometer 220.

We claim:

1. A variable capacity rotary screw compressor comprising: a housinghaving a pair of parallel cylindrical bores, a pair of oppositelyrotating constant mesh rotors positioned in said bores and definingworking chambers, a low pressure fluid chamber in fluid communicationwith said working chambers at one end of said rotors, a dischargechamber in fluid communication with said working chambers at the otherend of said rotors, a blind bore in each of said rotors, a plurality ofradial ports in said rotors, said ports lying in a common plane, a highpressure fluid chamber connected to each of said blind bores, cylindermeans in each of said bores for selectively advancing or retracting thepoint of admission of high pressure fluid to said ports in said rotors,a slide valve mounted in said housing for controlling the low pressurefluid cut off between minimum and maximum capacity positions withrespect to said rotors, and electrohydraulic means connected to respondto the position of the slide valve for controlling said high pressurefluid control means.

2. A variable capacity rotary screw compressor comprising:

a housing having a low pressure chamber and a high pressure chamber,

means for separating said high pressure chamber into a high pressurefluid inlet chamber and a high pressure discharge chamber,

a low pressure fluid inlet port connected to said low pressure chamber,

a high pressure fluid inlet port connected to said high pressure chamberand a fluid discharge port connected to said high pressure dischargechamber,

a pair of oppositely rotating constant mesh rotors having helical landsand intervening grooves defining working chambers, said rotors beingmounted within said housing to provide pumping and compressing actionand being connected at the inlet end to said low pressure chamber and atthe outlet end to said discharge chamber,

means at the outlet end for connecting the high pressure chamber to theworking chambers defined by said rotors,

a slide valve mounted within said housing for movement between minimumand maximum capacity positions with respect to said rotors,

and electrohydraulic means connected to respond to the position of saidslide valve and being operatively connected to said high pressure fluidconnecting means to change the angular point of admission of highpressure fluid to the working chambers in relation to the minimum andmaximum capacity positions of said slide valve.

3. The compressor according to claim 2 wherein said connecting meansincludes a pair of angularly offset ports for each of said rotorsconnecting the high pressure chamber to said rotors and a valve membercorresponding to each of said ports mounted for movement between openand closed positions with respect to the corresponding port andincluding means for connecting said valve members to saidelectrohydraulic means whereby one of said ports in each of said pairsof ports is opened when the slide valve is in the minimum capacityposition and the other of each of said pairs of bores is open when theslide valve is in the maximum capacity position.

4. A variable capacity multiple inlet rotary screw compressorcomprising:

a housing having a low pressure suction inlet port, a high pressuresuction inlet port and a discharge port,

a pair of oppositely rotating constant mesh rotors defining workingchambers within said housing, said rotors being positioned in saidhousing to provide pumping and compressing action and being connected todraw fluid from the low pressure inlet port and said high pressure inletport and to discharge the compressed fluid to the discharge port,

means mounted within said housing for controlling the low pressuresuction inlet fluid cut off for said rotors,

means for varying the point of admission of the high pressure inletfluid from said high pressure inlet port,

and electro-hydraulic means connected to respond to the position of thecut off means for controlling the high pressure varying means wherebysaid high pressure suction fluid is admitted to the working chamberssimultaneously with or just after low pressure suction fluid cut off.

5. The compressor according to claim 4 wherein said rotors have helicallands and intervening grooves having wrap angles of less than 360.

6. The compressor according to claim 4 wherein said high pressurevarying means comprises a number of plug valve assemblies operativelyconnected to said electro-hydraulic means for connecting the highpressure suction inlet port to said working chambers.

7. The compressor according to claim 6 wherein said plug valveassemblies include a number of ports angularly displaced in a planeradially outwardly from the axis of rotation of said rotors and acorresponding num ber of valve members positioned for movement intoengagement with said ports for selectively connecting said ports to theworking chambers defined by said rotors.

8. The compressor according to claim 4 wherein each of said rotorsincludes a bore and a number of radially extending ports connecting saidbore to said working chambers, and said varying means comprises acontrol valve assembly mounted in said bore in each of said rotors tocontrol the point of communication of said bore with the workingchambers.

9. The compressor according to claim 8 wherein each of said controlvalve assemblies includes a hollow cylinder positioned in said bore ineach of said rotors, said cylinder including an arcuate slot positionedto intersect the radial plane of said ports and including means formoving said cylinder axially in the bore of said rotors to angularlydisplace the point of intersection of said ports with said arcuate slot.

10. A variable capacity rotary screw compressor comprising: a housinghaving a low pressure suction fluid chamber, a high pressure suctionfluid chamber and a fluid discharge chamber, a low pressure fluid inletport connected to said low pressure chamber, a high pressure fluid inletport connected to said high pressure chamber and a discharge portconnected to said discharge chamber, a pair of oppositely rotatingconstant mesh rotors having helical lands and interven ing groovesdefining working chambers, said rotors being mounted within said housingto provide pumping and compressing action, said working chambers beingconnected at one end to draw fluid from the low pressure chamber and atthe other end to discharge compressed fluid to the discharge chamber,means for selectively providing fluid communication between said highpressure fluid chamber and said working chambers, a slide valve movablymounted within said housing and including cut off means for controllingthe ca pacity of said compressor, and means connected to said slidevalve and to the high pressure fluid communicating means for controllingthe point of admission of high pressure fluid from said high pressurechamber to said working chamber in relation to the position of saidslide valve.

11. The compressor according to claim 10 wherein said controlling meanscomprises a control valve assembly mounted in each of said rotors.

12. The compressor according to claim 11 wherein said rotors eachincludes a bore and a number of radially extending ports connecting saidbores to said intervening grooves and said control valve assemblyincludes a hollow cylinder positioned in said bore in each of saidrotors, said cylinder including an arcuate slot positioned to intersectthe radial plane of said radial ports in each of said rotors and furtherincluding means for moving said cylinder axially in said bore of saidrotors to angularly displace the point of intersection of said portswith said arcuate slot.

13. The compressor according to claim wherein said high pressure fluidcommunication means comprises a number of ports in said rotor connectingsaid high pressure chamber to one end of said working chambers and saidcontrolling means includes a number of valve members corresponding tosaid ports and being mounted for movement between open and closedpositions with respect to said ports.

14. The compressor according to claim 13 wherein said controlling meansincludes a first variable resistor connected to respond to the movementsof the slide valve, a pair of bridge balancing relays connected to saidfirst variable resistor to sense the change in the position of saidfirst variable resistor, a second and a third variable resistorconnected to each of said bridge balancing ralays, means for connectingsaid second and third variable resistors to respond to movements of saidselectively providing means and hydraulic circuit means connected tosaid bridge balancing relays to control said selectively providing meansto vary the point of fluid communication between said high pressurefluid chamber and said working chambers.

1. A variable capacity rotary screw compressor comprising: a housinghaving a pair of parallel cylindrical bores, a pair of oppositelyrotating constant mesh rotors positioned in said bores and definingworking chambers, a low pressure fluid chamber in fluid communicationwith said working chambers at one end of said rotors, a dischargechamber in fluid communication with said working chambers at the otherend of said rotors, a blind bore in each of said rotors, a plurality ofradial ports in said rotors, said ports lying in a common plane, a highpressure fluid chamber connected to each of said blind bores, cylindermeans in each of said bores for selectively advancing or retracting thepoint of admission of high pressure fluid to said ports in said rotors,a slide valve mounted in said housing for controlling the low pRessurefluid cut off between minimum and maximum capacity positions withrespect to said rotors, and electrohydraulic means connected to respondto the position of the slide valve for controlling said high pressurefluid control means.
 2. A variable capacity rotary screw compressorcomprising: a housing having a low pressure chamber and a high pressurechamber, means for separating said high pressure chamber into a highpressure fluid inlet chamber and a high pressure discharge chamber, alow pressure fluid inlet port connected to said low pressure chamber, ahigh pressure fluid inlet port connected to said high pressure chamberand a fluid discharge port connected to said high pressure dischargechamber, a pair of oppositely rotating constant mesh rotors havinghelical lands and intervening grooves defining working chambers, saidrotors being mounted within said housing to provide pumping andcompressing action and being connected at the inlet end to said lowpressure chamber and at the outlet end to said discharge chamber, meansat the outlet end for connecting the high pressure chamber to theworking chambers defined by said rotors, a slide valve mounted withinsaid housing for movement between minimum and maximum capacity positionswith respect to said rotors, and electrohydraulic means connected torespond to the position of said slide valve and being operativelyconnected to said high pressure fluid connecting means to change theangular point of admission of high pressure fluid to the workingchambers in relation to the minimum and maximum capacity positions ofsaid slide valve.
 3. The compressor according to claim 2 wherein saidconnecting means includes a pair of angularly offset ports for each ofsaid rotors connecting the high pressure chamber to said rotors and avalve member corresponding to each of said ports mounted for movementbetween open and closed positions with respect to the corresponding portand including means for connecting said valve members to saidelectrohydraulic means whereby one of said ports in each of said pairsof ports is opened when the slide valve is in the minimum capacityposition and the other of each of said pairs of bores is open when theslide valve is in the maximum capacity position.
 4. A variable capacitymultiple inlet rotary screw compressor comprising: a housing having alow pressure suction inlet port, a high pressure suction inlet port anda discharge port, a pair of oppositely rotating constant mesh rotorsdefining working chambers within said housing, said rotors beingpositioned in said housing to provide pumping and compressing action andbeing connected to draw fluid from the low pressure inlet port and saidhigh pressure inlet port and to discharge the compressed fluid to thedischarge port, means mounted within said housing for controlling thelow pressure suction inlet fluid cut off for said rotors, means forvarying the point of admission of the high pressure inlet fluid fromsaid high pressure inlet port, and electro-hydraulic means connected torespond to the position of the cut off means for controlling the highpressure varying means whereby said high pressure suction fluid isadmitted to the working chambers simultaneously with or just after lowpressure suction fluid cut off.
 5. The compressor according to claim 4wherein said rotors have helical lands and intervening grooves havingwrap angles of less than 360*.
 6. The compressor according to claim 4wherein said high pressure varying means comprises a number of plugvalve assemblies operatively connected to said electro-hydraulic meansfor connecting the high pressure suction inlet port to said workingchambers.
 7. The compressor according to claim 6 wherein said plug valveassemblies include a number of ports angularly displaced in a planeradially outwardly from the axis of rotation of said rotors and acorresponding number of valve members positioned for movement iNtoengagement with said ports for selectively connecting said ports to theworking chambers defined by said rotors.
 8. The compressor according toclaim 4 wherein each of said rotors includes a bore and a number ofradially extending ports connecting said bore to said working chambers,and said varying means comprises a control valve assembly mounted insaid bore in each of said rotors to control the point of communicationof said bore with the working chambers.
 9. The compressor according toclaim 8 wherein each of said control valve assemblies includes a hollowcylinder positioned in said bore in each of said rotors, said cylinderincluding an arcuate slot positioned to intersect the radial plane ofsaid ports and including means for moving said cylinder axially in thebore of said rotors to angularly displace the point of intersection ofsaid ports with said arcuate slot.
 10. A variable capacity rotary screwcompressor comprising: a housing having a low pressure suction fluidchamber, a high pressure suction fluid chamber and a fluid dischargechamber, a low pressure fluid inlet port connected to said low pressurechamber, a high pressure fluid inlet port connected to said highpressure chamber and a discharge port connected to said dischargechamber, a pair of oppositely rotating constant mesh rotors havinghelical lands and intervening grooves defining working chambers, saidrotors being mounted within said housing to provide pumping andcompressing action, said working chambers being connected at one end todraw fluid from the low pressure chamber and at the other end todischarge compressed fluid to the discharge chamber, means forselectively providing fluid communication between said high pressurefluid chamber and said working chambers, a slide valve movably mountedwithin said housing and including cut off means for controlling thecapacity of said compressor, and means connected to said slide valve andto the high pressure fluid communicating means for controlling the pointof admission of high pressure fluid from said high pressure chamber tosaid working chamber in relation to the position of said slide valve.11. The compressor according to claim 10 wherein said controlling meanscomprises a control valve assembly mounted in each of said rotors. 12.The compressor according to claim 11 wherein said rotors each includes abore and a number of radially extending ports connecting said bores tosaid intervening grooves and said control valve assembly includes ahollow cylinder positioned in said bore in each of said rotors, saidcylinder including an arcuate slot positioned to intersect the radialplane of said radial ports in each of said rotors and further includingmeans for moving said cylinder axially in said bore of said rotors toangularly displace the point of intersection of said ports with saidarcuate slot.
 13. The compressor according to claim 10 wherein said highpressure fluid communication means comprises a number of ports in saidrotor connecting said high pressure chamber to one end of said workingchambers and said controlling means includes a number of valve memberscorresponding to said ports and being mounted for movement between openand closed positions with respect to said ports.
 14. The compressoraccording to claim 13 wherein said controlling means includes a firstvariable resistor connected to respond to the movements of the slidevalve, a pair of bridge balancing relays connected to said firstvariable resistor to sense the change in the position of said firstvariable resistor, a second and a third variable resistor connected toeach of said bridge balancing ralays, means for connecting said secondand third variable resistors to respond to movements of said selectivelyproviding means and hydraulic circuit means connected to said bridgebalancing relays to control said selectively providing means to vary thepoint of fluid communication between said high pressure fluid chamberand said working chambers.